Organotin compounds have been manufactured and applied as antifouling agents on vessel hulls on a large scale since the late 1960s. TBT is regarded as the most toxic organotin to marine organisms and has been referred to as the most toxic substance to have been deliberately released in the marine environment. Owing to many of the deleterious effects of TBT on non-target organisms (principally marine snails and oysters) many countries imposed a partial ban on its use in the 1980's. Recently, the Marine Environmental Protection Committee of the International Maritime Organisation (IMO) has recommended a ban on the application of TBT-based antifouling paints from 2003 and a total ban from 2008.

TBT is hydrophobic and has a half life in the water column of about six hours since it rapidly partitions to either the surface microlayer or in to sediments. In sediments, however, the half life for TBT is longer and has been estimated at 3.5 years(45). The deleterious, sublethal effects of TBT on marine organisms (especially molluscs) are well documented. TBT is an endocrine disrupter and its most widely documented effect is imposex (ie the imposition of male genitalia in female snails) which is known to interfere with reproduction at nanogram per litre levels. TBT has also been implicated in mollusc growth abnormalities (ie shell thickening) and is an immunosuppresent. Due to the ability of TBT to partition in to sediments, benthic organisms are potentially at the greatest risk of TBT exposure. Molluscs inhabiting intertidal locations near to centres of high shipping activity have also been affected(46) .

Only three of the stations sampled in the baseline SUSDEV 21 surveys reported TBT at above the detection limit of 20 ug Sn kg-1 (VS3 in Victoria Harbour, WS1 south of Tsing Yi and NS4 located in Urmston Road). TBT was detected in VS3 sediments during each sampling period (20.8, 36.2, 34.6 and 46.6 mg ug Sn kg-1 during December 1998, January, June and August 1999, respectively. TBT in sediments at station WS1 was also recorded above detection limits in all four sampling periods (81.8, 103.0, 107.0 and 93.0 ug Sn kg-1, respectively). TBT was recorded only once above the detection limit (39.2 ug Sn kg-1) in sediments from NS4 during August 1999.

TBT was only rarely recorded above the detection limit in tissues of the biota sampled. The maximum number of stations showing biota with TBT contamination above detection limits was 12% during the initial trawl survey conducted in December 1998 (dry season). Trawls during January (dry season), August (wet season) and September (wet season) 1999 detected TBT in biota 3%, 3% and 0% of stations, respectively. Based on the low rates of detection for tissue TBT in biota in the baseline survey, it would appear, therefore, that TBT contamination of marine life in local waters away from marinas, port facilities and typhoon shelters is low and not a direct threat to fisheries or ecological resources. Extremely low levels of TBT (ng L-1) can, however, induce reproductive abnormalities (imposex) in marine snails. Such low concentrations are difficult to measure accurately and are below the detection limits presently used in most laboratories involved in TBT analysis throughout the world.